This invention relates generally to the casting of metals. More specifically, it relates to the preparation of shell molds used in the casting of metal components.
Ceramic shell molds are used in the investment-casting of metals, to contain and shape the metal in its molten state. The strength and integrity of the mold are very important factors in ensuring that the metal part has the proper dimensions. These shell mold characteristics are especially critical for manufacturing high performance components, such as superalloy parts used in the aircraft and power generation industries.
Very high casting temperatures, such as, in the range of about 1500xc2x0 C. to 1750xc2x0 C., are sometimes employed. Many conventional shell molds do not exhibit sufficient strength at those temperatures. The molds become susceptible to bulging and cracking when they are filled with the molten metal. Bulging can also occur when very large parts are being cast at even lower temperatures. Bulging can alter the dimensions of the mold, thereby causing undesirable variation in the component being cast. Cracking could result in failure of the mold as the molten material runs out of it.
Greater strength is required for shell molds used at very high casting temperatures, or for those used to cast very large parts. The problem is addressed by J. Lane et al. in U.S. Pat. No. 4,998,581. In that disclosure, shell molds are strengthened by wrapping a fibrous reinforcing material around the shell mold as it is being made. In preferred embodiments, the reinforcing material is said to be an alumina-based or mullite-based ceramic composition having a specific, minimum tensile strength. The reinforcing material is apparently wrapped in spiral fashion around the shell mold with a tension sufficient to keep it in place as ceramic layers are applied to the mold to build it up to its desired thickness.
U.S. Pat. No. 4,998,581 appears to provide answers to some of the problems described above. However, there appear to be some considerable disadvantages in practicing the invention disclosed in that patent. For example, mullite-based materials are difficult to produce without second phase inclusions of either silica- or alumina-containing compounds. These inclusions can degrade the physical properties of the mold. In addition, many of the reinforcing materials employed in U.S. Pat. No. 4,998,581 have thermal expansions much less than the mold. These large thermal expansion differences will make fabrication of a crack-free mold more difficult.
Therefore, further improvements in the properties of shell would be welcome in the art. The shell molds should have the strength to withstand high metal-casting temperatures, and should be suitable for casting large parts. The molds should also be dimensionally stable at elevated temperatures, and throughout various heating/cooling cycles. Moreover, if the molds are to be improved by the use of reinforcing materials, such materials should be flexible enough, before being fired, to satisfy the shape requirements for the mold, especially when intricate metal components are being cast. Finally, the preparation of improved shell molds should be economically feasible and not require the use of a significant amount of additional equipment.
In one aspect, the invention comprises a ceramic casting shell mold having a pre-selected shape. The shell mold comprises alternate, repeating layers of a ceramic coating material and a ceramic stucco, defining a total thickness of the shell mold; and a ceramic-based reinforcing sheet disposed in the alternate, repeating layers of coating material and stucco at an intermediate thickness. The ceramic-based reinforcing sheet comprises a one-piece monolithic, integral body, which comprises a pattern of holes that enhance bonding between the ceramic-based reinforcing sheet and adjacent ones of the alternate, repeating layers of ceramic coating material. The ceramic-based reinforcing sheet conforms to the shape of the mold and providing structural reinforcement to the mold.
Another aspect of the invention provides a method for making a ceramic casting shell mold. The method comprises applying a reinforcing layer to a ceramic layer-surface of a partial shell mold that is being formed by an investment casting process; completing the shell mold by applying additional ceramic layers over the reinforcing layer; wherein the reinforcing layer comprises a pattern of holes that enhance bonding between the reinforcing layer and adjacent ones of the ceramic layer-surface and the additional ceramic layers; and firing the shell mold at an elevated temperature.